2,6-Naphthalenedicarboxylic acid (2,6-NDA) is a monomer useful for the preparation of high performance polymeric materials such as polyesters and polyamides. Polyethylene 2,6-naphthalate (PEN) is one such high performance polymer and it is prepared, for example, by the condensation of either 2,6-naphthalenedicarboxylic acid or dimethyl-2,6-naphthalenedicarboxylate with ethylene glycol. Fibers and films made from PEN have improved strength and thermal properties relative to, for example, fibers and films made from polyethylene terephthalate. High strength fibers made from PEN can be used to make tire cord, and films made from PEN are advantageously used to manufacture magnetic recording tape and electronic components. Also, because of its superior resistance to gas diffusion, and particularly to the diffusion of carbon dioxide, oxygen and water vapor, films made from PEN are useful for manufacturing food containers, particularly so-called "hot fill" type food containers.
In order to prepare high quality PEN suitable for the aforementioned applications, it is desirable to start with purified 2,6-naphthalenedicarboxylic acid or purified dimethyl-2,6-naphthalenedicarboxylate (DM-2,6-NDC). Since dimethyl-2,6-naphthalenedicarboxylate is typically prepared by the esterification of 2,6-naphthalenedicarboxylic acid using methanol, a purer form of 2,6-naphthalenedicarboxylic acid provides for purer dimethyl-2,6-naphthalenedicarboxylate. It is therefore advantageous to have the highest purity 2,6-naphthalenedicarboxylic acid.
2,6-Naphthalenedicarboxylic acid is most conveniently prepared by the liquid phase, heavy metal catalyzed oxidation of 2,6-dimethylnaphthalene using molecular oxygen, and particularly air, as the source of oxygen for the oxidation reaction. During this oxidation, the methyl substituents on the naphthalene ring of 2,6-dimethylnaphthalene are oxidized to carboxylic acid substituents. Processes for oxidizing 2,6-dimethylnaphthalene to 2,6-naphthalenedicarboxylic acid by such a liquid phase reaction are known. For example, U.S. Pat. No. 3,870,754 to Yamashita et al. discloses a process for oxidizing 2,6-dimethylnaphthalene in acetic acid solvent using molecular oxygen and a catalyst containing cobalt, manganese and bromine components, wherein the mole ratio of 2,6-dimethylnaphthalene to the acetic acid solvent is maintained at no greater than 1:100 and preferably no greater than 1:200.
U.S. Pat. No. 3,856,805 to Yamashita et al. discloses a process for oxidizing 2,6-dimethylnaphthalene in acetic acid using molecular oxygen and catalyzed by cobalt, manganese and bromine catalyst compounds at a reaction temperature no greater than 170.degree. C. It is taught therein that oxidation temperatures exceeding 170.degree. C. (338.degree. F.) produce an extreme amount of by-products and coloration of the 2,6-naphthalenedicarboxylic acid. It is also taught that at temperatures exceeding 180.degree. C., black "carbido-like" products are formed, and that it is impossible to obtain the intended naphthalenedicarboxylic acid in high yield. However, we have determined that low reaction temperatures do not provide for sufficiently reduced levels of 2-formyl-6-naphthoic acid. Additionally, lower reaction temperatures generally mean lower reaction rates, whereas rapid reaction rates are desirable for commercial processes.
During the liquid phase oxidation of 2,6-dimethylnaphthalene to 2,6-naphthalenedicarboxylic acid using a catalyst comprising cobalt, manganese and bromine components various side products are usually produced. For example, trimellitic acid (TMLA) is produced by the oxidation of one of the rings of the 2,6-dimethylnaphthalene molecule. 2-Formyl-6-naphthoic acid (FNA), a result of incomplete oxidation, is also produced. Bromination of the naphthalene ring during the oxidation reaction results in the formation of bromo naphthalenedicarboxylic acid (BrNDA). Additionally, loss of one methyl (or carboxylic acid) substituent during the oxidation reaction results in the formation of 2-naphthoic acid (2-NA). These side products, as well as a collection of other unidentified side products, are undesirable because, to some extent, they contaminate the 2,6-naphthalenedicarboxylic acid product, and their formation represents a reduced yield of the desired 2,6-naphthalenedicarboxylic acid. Additionally, trimellitic acid deactivates the oxidation catalysts by complexing to cobalt and manganese. Therefore, an oxidation process that produces trimellitic acid is self-deactivating. Finally, the contamination of the 2,6-naphthalenedicarboxylic acid by the side products produced during the oxidation reaction is a major problem because 2,6-naphthalenedicarboxylic acid, due to its high insolubility in ordinary solvents such as water, acetic acid, and aliphatic as well as aromatic hydrocarbons, is very difficult to purify by standard purification treatments such as recrystallization or adsorption. Therefore, it is important to produce 2,6-naphthalenedicarboxylic acid with low levels of these aforementioned impurities, and particularly trimellitic acid and 2-formyl-6-naphthoic acid.
The art needs a process for the continuous, liquid-phase oxidation of 2,6-dimethylnaphthalene suitable for large-scale commercial operations and that can produce 2,6-naphthalenedicarboxylic acid in high yield and having low levels of impurities such as trimellitic acid, 2-formyl-6-naphthoic acid, bromo naphthalenedicarboxylic acid as well as other impurities. The present invention provides such a process.